Double acting piston pumps having a cylinder accommodating a pair of pistons that are reciprocated by an eccentric cam roller for pumping air are known. H. A. Berglund and D. F. Thomas disclose in U.S. Pat. No. 3,776,667 a double acting pump having a pair of cylinders accommodating a pair of pistons for compressing air and liquid, and mixing the air with the liquid. An eccentric cam associated with the pistons operates to simultaneously reciprocate the pistons in their respective cylinders. Poppit-type check valves associated with each cylinder operate to control the flow of air and liquid through the respective cylinders during the reciprocation of the pistons in the cylinders. The pump is made from a number of separable parts so that it can be readily disassembled for cleaning and repair. The parts must be fabricated and machined which increases the manufacturing, labor and costs of the pump.